Frame removal mechanism for token ring networks

ABSTRACT

In accordance with principles of the invention, a frame removal mechanism for token ring networks is disclosed. The disclosed system uses strip delimiter frames. The system transmits one or more information frames onto a computer network. The system further transmits one or more strip delimiter frames onto the computer network after the information frame transmission. The system detects strip delimiter frames on the computer network, and strips information frames received from the computer network after the information frame transmission is begun and until one of the strip delimiter frames is detected.

This is a continuation of Ser. No. 07/880,805 filed on May 8, 1992, U.S.Pat. No. 5,477,540, which is a divisional of Ser. No. 07/678,888 filedon Mar. 28, 1991, now abandoned, which is a continuation of Ser. No.07/577,828 filed Sep. 4, 1990, now abandoned, which is a continuation ofSer. No. 07/483,296 filed Feb. 20, 1990, now abandoned, which is acontinuation of Ser. No. 07/231,773 filed Aug. 12, 1988, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of computer networkscomprising a plurality of stations which communicate data in a ringusing a token-based protocol and more specifically to a mechanism forremoving data from the network.

2. Description of the Prior Art

A computer network enables various devices, such as processors, massstorage devices, and printers, to communicate with each other over highspeed communication links. The devices are connected, that is,interfaced, to the network, and thus transfer data to and from thenetwork, through stations. The stations are interconnected by thecommunication links.

One example of such a network is a Local Area Network (LAN). A LANallows high-bandwidth communication among devices within a limitedgeographic area. The communication links are typically composed of fiberoptics, coaxial cable or twisted conductor pairs.

LAN's may be configured using of a number of topologies, for example,buses or rings. In a network configured as a bus, data is communicatedin either direction from the originating station. In a networkconfigured as a ring, network stations transmit data around the ring inone direction. Thus when a station transmits data onto the ring the datatravels around the ring from station to station, with each stationreceiving the data from the preceding station and repeating it to thesucceeding station, until the data reaches the station that is toreceive it. In a well-known ring network the receiving, or destination,station also repeats the data to its downstream successor and the datathus continues around the ring to the originating, or source, station.

Network stations transmit and receive data in the form of frames. Theframes contain, in addition to the data, addresses, an error detectionsequence and status flags, for example, a flag indicating receipt of theframe by the intended receiving, or destination, station. The frameaddresses identify the source station and the destination station orstations.

Every station and every attached device are each identified by a uniqueaddress. Thus each station has associated with it its uniquestation-address and the addresses of its attached devices. The stationsmay have other addresses, for example, logical addresses, associatedwith them, also. A frame directed from a source station to a singledestination station thus contains the address of the source station and,for example, the address of a specific device attached to thedestination station. Each station may maintain a list of its associatedaddresses, such that the station receives the frames containing any oneof its addresses.

A destination station, in addition to repeating the frame to itssucceeding station, copies the frame for use by the appropriatedevice(s) associated with the station. The destination station may alsoset the applicable status flags indicating receipt of the frame.

When the frame has returned to the source station, the station,recognizing the source address in the frame as its own, removes, orstrips, the frame from the ring data stream. After a transmission, thefirst frames received by the source station over the ring are the framesit transmitted, assuming proper operation of the stations on the ring.

The stations operate in accordance with communication protocols whichfacilitate the orderly transfer of the frames over the communicationlinks. One type of communication protocol is a token-ring system. Suchsystems use a token, that is, a specific string of bits, to indicatethat a transmitting station has completed its transmission. A succeedingstation may then, following the dictates of the protocol, begintransmitting frames upon receipt of the token.

In a token-ring system a station may not transmit data over the ringunless it holds the token. Thus if a station has frames to transmitaround the ring it first removes the token from the ring data stream,that is, it "captures" the token, when it receives it from the precedingstation. The capturing station then begins its frame transmission. Whenthe station is finished with the frame transmission it retransmits thetoken, effectively releasing it. The succeeding stations thereaftertransfer the token around the ring until a station captures it in orderto make a transmission.

Two or more such rings, or more generally, two or more LAN's, may beconnected by a bridge, that is, a special purpose station simultaneouslyconnected to both LAN's, to form an extended LAN. The bridge thusfacilitates communication between a transmitting station on, forexample, one of the rings and a receiving station on another ring. Itessentially acts as the destination station on the ring containing thetransmitting station and thus copies the frames directed to a secondring, while also repeating these frames on the source station ring. Italso acts as a transmitting station on the second ring and transmits thecopied frames in that ring after capturing the ring's token. The secondring may contain the destination station, or it may be an intermediatering which contains a bridge to another LAN, which may, in turn, containthe destination station or be another intermediate LAN.

There are two methods of bridge operation, namely, transparent andnon-transparent. A bridge operating transparently transmits the framesoriginated by the source station without significantly altering them,that is, without changing any portion of the frame which is protected bythe frame's error detection sequence. Alternatively, a bridge operatingnon-transparently significantly alters the frames originated by thesource station before transmitting them to the connected LAN. Anon-transparent bridge may, for example, alter the source address to theaddress of the bridge on the second ring and/or it may alter certainother information in the frame.

The non-transparent bridge must recalculate and replace the errordetection sequence contained in each of the frames to prevent the framealterations from being treated as errors. However, if the bridgeincorrectly copies a frame or inadvertently copies an erroneous frame,alters it and then recalculates the error detection sequence, it mayrender the frame errors undetectable. The integrity of the frameinformation is considered to be thus reduced.

The bridge, regardless of the mode in which it operates, must strip fromthe second ring the frames it transmits. Typically, a station"recognizes" the source address in that frame as its own address andstrips the frame. A bridge which leaves the addresses unaltered, andwhich is not the original source station, may not recognize the sourceaddress in the frames it has transmitted over the second ring. Thebridge is capable of transmitting frames which were originallytransmitted by one of many source stations on one or more differentLAN's. When the frames return to the bridge over the second ring, it maynot have time to compare the frame source address with the addresses ofthe many source stations on the various LAN's before the decision tostrip the frame must be made. Thus the bridge could repeat a frame thatshould be stripped by it. Accordingly, there must be a method for abridge to determine which frames to strip.

One possible method for stripping such frames involves the bridgemarking the last frame in the sequence of transmitted frames, e.g., bysetting a flag in the frame. The bridge then, after a transmission,strips the frames it receives over the ring until it detects the setflag in one of the frames. If, however, the frame containing the flag islost or corrupted, making the set flag undetectable, the bridge maycontinue to strip the frames it receives over the ring until it issignalled to stop, for example, by the expiration of a locally kepttimer. The bridge thus strips frames originated by other stations on theextended LAN, preventing the frames from reaching their intendeddestinations. If the flag is a single-bit flag, a bit-error in the framemay result in the flag appearing re-set, and thus, the bridge will stopstripping before it has removed all of its frames.

A non-bridge station which connects many devices to the LAN may alsohave problems determining which frames it should strip. A station mustdetermine whether to strip a frame within the time it takes the frameaddresses to pass through the station. Thus the station may not havetime to match the frame source address with one of its many associatedaddresses, and another method for determining which frames to strip mustbe used.

SUMMARY OF THE INVENTION

The invention provides a new and improved station and a method forenabling the station to strip the frames it transmits.

In brief summary, a station incorporating the invention transmits asequence of one or more frames on a ring, without setting any flags inthe frames, and keeps a count of the frames transmitted. At the end ofthe transmitted sequence it also originates on to the ring one or morecoded marker frames. After the transmission is begun, the station stripsfrom the ring the frames it receives, decrementing its count for eachproperly stripped frame, until either its count is reduced to zero orone of its marker frames is detected.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is pointed out with particularity in the appended claims.The above and further advantages of this invention may be betterunderstood by referring to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 depicts a functional block diagram of an extended LAN;

FIG. 2 depicts an illustration of an information frame used in theextended LAN shown in FIG. 1; and

FIGS. 3A-3C comprise a flow chart depicting the operation of bridges onthe extended LAN shown in FIG. 1 operating in accordance with thepreferred embodiment.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

With reference to FIG. 1, a ring 10, that is, a LAN configured in a ringtopology, includes a plurality of stations 11A through 11F connected bycommunication links 12A through 12F. The stations 11 interface one ormore devices 14A through 14F to the ring. The devices 14 may be ofdiverse types, including computers, mass storage devices,telecommunications links, printers, and so forth, all of which maytransmit information to, or receive information from, other devices inthe ring 10 over communication links 12.

Similarly a ring 20 includes a plurality of stations 21A through 21Einterconnected by communications links 22A through 22E, and bus 30includes a plurality of stations 31A through 31C. Each of the stations21 and 31 connects one or more devices identified generally as 24 or 34to the ring 20 and bus 30, respectively.

Ring 20 is interconnected with ring 10 by a bridge 15. It is alsointerconnected by a bridge 17 to another network (not shown). Thebridges are special purpose stations that are simultaneously stations oneach of the LAN's they interconnect. Bridge 16 interconnects bus 30 withring 10, thus stations on ring 20 can also communicate with stations onbus 30 via ring 10 using both bridges 16 and 15. The interconnectednetworks together form an extended LAN 8.

The stations 11 and 21, including the bridge-stations, transmitinformation over the respective communications links 12 and 22 in theform of frames, the structure of which will be described below inconnection with FIG. 2. Communication between two stations on the samering occurs when, for example, one station 11 transmits a frame toanother station 11. To accomplish this, the station 11 originating theframe transmits a signal comprising a bit stream forming the frame toanother station 11 (following the direction of the arrow) over thecommunications link 12 interconnecting the two stations. The station 11that receives a bit stream from the originating station 11 repeats theframe over the communications link 12 interconnecting it and the nextstation 11.

This process is repeated by each of the other stations 11 until theframe is returned to the originating station 11. Essentially, each ofthe stations receives a signal over a communications link 12interconnecting it and a preceding station 11, and transmits a signalover another communications link 12 interconnecting it and a succeedingstation 11. If a station 11 is an intended recipient of the frame, inaddition to repeating the frame to its downstream station 11, it alsoretains a copy of the frame for processing by one or more of the devices14 connected to it.

The determination of whether a station 11 or 21 may transmit a frame toa ring is based on whether the station holds a token. A token is aspecial sequence of bits which the stations transmit around the ring.If, for example, station 11 is to send a frame, when the token reachesit, it does not repeat the token, thereby capturing the token andbecoming its holder. The station 11 then transmits one or more frames.After completing the transmission of these frame(s), the station 11resumes transmission of the token around the ring in accordance with thering protocol, effectively releasing it to other stations on the ring.

When a station 11 receives a frame which it originated, other than thetoken, it strips the frame from the ring 10. Thus a frame is transmittedaround the ring, that is, repeated from station to station, only once.If the originating station does not strip one of its frames, theintended destination station may receive duplicates of the frame. Thedestination station receives these duplicate frames at a faster ratethan it ordinarily receives frames. Thus the station must process theseframes more quickly, resulting in additional station overhead andpossibly station errors.

Communication between two stations on different rings, for example, astation 21 on ring 20 and a station 11 on ring 10, is accomplished usingthe bridge 15. The bridge 15 acts as the intended receiving station onring 20 and copies the frame as well as repeating it to the next station21. The bridge 15 then transmits the copied frame on ring 10, after itcaptures the ring 10 token, and the intended receiving station 11 maythen copy the frame when it receives it over the ring 10. After theframe is transmitted around ring 10 it is received by the bridge 15 andthe bridge 15 then strips it. The operation of the bridge 15 isdiscussed in more detail with reference to FIGS. 3A-3C below.

As noted above, information is transferred in the form of frames, thestructure of which is shown in FIG. 2. With reference to FIG. 2, a frame40 is divided into several fields. The start of a frame is denoted by apreamble field 41A and a start frame delimiter field 41B which furtherdenotes the beginning of the portion of the frame containing the frameinformation. These fields are generally referred to as a start-of-framesequence 41.

Immediately following the start-of-frame sequence 41 is a frame controlfield 42 detailing whether the bit stream is a token or a frame and, ifit is a frame, indicating the type of frame, for example, an informationframe. The next two fields are address fields 43A-B, namely, adestination address field DA 43A whose contents identify the intendedrecipient(s) of the frame and a source address field SA 43B whosecontents identify the source station. The information field 44 followsthe address fields 43, and is in turn followed by a frame check sequencefield 45 containing an error detection sequence used by receivingstations to detect errors in any of the frame control field 42, theaddress fields 43 and the information field 44.

The end of the frame is denoted by an end-of-frame sequence 46. Thesequence 46 includes an end delimiter field 46A, which defines the endof a frame, and a frame status field 46B containing a set of statusflags. The status flags are conditioned by stations other than thesource station to provide status information to the source station. Thisinformation may include, for example, whether the frame was copied bythe intended destination station(s), whether any errors have beendetected, etc.

If a station 11 or 21 recognizes the contents of the frame destinationaddress field 43A as one of its own addresses, that is, either itsstation-address or one of its other associated addresses, it copies theframe for processing by its attached devices 14 or 24. It also repeatsthe frame to the next station in the ring as set forth above.

When a frame returns to its source station, the latter recognizes thecontents of the frame source address field 43B as its own address, andstrips the rest of the frame from the ring. When a frame is thusstripped, the start-of-frame sequence 41, the frame control field 42 andpart of the address fields 43 remain. These fragments are ignored by thestations on the ring because they do not form a complete frame. They arestripped from the ring when they encounter a station in the act oftransmitting.

If a ring is improperly operating, for example, if the token is lost ora break develops in the ring, the stations 11 and 21 begin operating inan error detection and recovery mode defined by the network protocol.The error detection and recovery mode may require all stations toactively participate or it may require the participation of only a fewof the stations. Two widely used network protocols include errordetection and recovery techniques involving the use of special-purposeframes, namely claim token frames and beacon frames.

The claim token frames are used when the ring is initialized or when thering token is lost and the ring must thus be re-initialized. Basically,each station 11 and 21 has associated with it a priority for errordetection and recovery purposes. When the token is lost each stationcontinuously transmits claim token frames containing its priority untilit receives another station's claim token frame. The receiving stationthen compares the priority in the frame with its own priority, and ifthe station has a lower priority than that contained in the frame itrepeats the frame. Otherwise, it strips the frame from the ring andtransmits another of its own claim token frames. When a station receivesits own claim token frame, which indicates that the station has thehighest priority, it originates the network token.

The beacon frames are used when a break in the ring or a substantialring reconfiguration is suspected. Basically each station continuouslytransmits its own beacon frames to downstream stations. A downstreamstation, upon receipt of a beacon frame, stops transmitting its ownbeacon frames and repeats the received beacon frames to its successorstation. When a station receives it own beacon frames, it is assumedthat the network is properly configured and ring re-initialization,involving the use of the claim token frames as described above, isbegan.

The detailed operation of the bridges 15-17 will now be described inconnection with flow charts in FIGS. 3A-3C. The operations may beperformed using hardware, software and/or firmware. It will beappreciated by those skilled in the art that non-bridge stationsconnecting a number of attached devices to the LAN operate in a mannerwhich is similar to that of a bridge. Thus much of the discussion belowrelating to bridges applies, as well, to any station on the network.

With reference to FIG. 3A, a bridge, for example bridge 15, detecting aframe 40 originating on, for example, ring 10, determines if theintended destination station is on another LAN which is accessible fromthe bridge (step 50). If it is, the bridge 15 retains a copy of theframe 40 for transmission to the connected LAN, ring 20, and thenrepeats the frame on ring 10 (step 51). Alternatively, the bridge 15 maydiscern if the intended destination station is on ring 10 and, if it isnot, copy the frame for transmission to the connected LAN, whilerepeating it on the ring 10.

After the bridge 15 copies one or more frames, it transmits the copiedframes onto the ring 20 once it captures the ring 20 token (steps53-54). The bridge 15 continues to transmit copied frames onto the ring20 until either all the copied frames it holds for transmission on ring20 are transmitted or it is forced by network protocol rules to stoptransmitting and release the ring 20 token (steps 56-57). If the bridge15 has not transmitted all of the copied frames before it ends thecurrent transmission, it resumes transmission of copied frames when itagain captures the ring 20 token (step 60).

Each time the bridge 15 transmits a frame onto ring 20 it increments aninternal frame count by one (step 55), essentially keeping a runningcount of the number of frames transmitted. The count may be soincremented either at the beginning of the frame, for example, when thestart frame delimiter field 418 (FIG. 2) is transmitted, or at the endof the frame, for example, when the end delimiter field 46A istransmitted.

At the close of the bridge's transmission of frames over the ring 10,the bridge 15 transmits onto the ring 20 one or more marker, or stripdelimiter, frames marking the end of the transmission, incrementing theframe count by one for each of these frames (step 59). The count neednot be incremented for these frames. However, in the preferredembodiment it is so incremented to keep an exact count of the framestransmitted by the bridge 15.

The strip delimiter frames are coded information frames specifying atleast the frame type, that is, delimiter frame, the address of theoriginating bridge station, and the frame status, making each frame afull, or valid, frame. The number of strip delimiter frames transmittedis determined by the bridge 15. Redundant strip delimiter frames may beused to ensure that at least one delimiter frame will be transmittedaround the ring 20 and returned to the bridge 15, thus giving thenetwork a certain immunity to error. For further impunity to errors, thestrip delimiter frames may be protected by a frame check sequence 45(FIG. 2). Using the frame check sequence enables the source station todetermine if the delimiter frame contains any errors. Errors in theframe may indicate a problem with one or more of the network stationsand, as discussed below with reference to FIGS. 3A-3C, indicate to thesource station that it is to continue stripping.

It will be appreciated by those skilled in the art that the framestripping operation by a bridge, or any source station, must be quicklyperformed. As set forth above, a station typically determines whether ornot to strip a frame by looking at the frame source address field 43B.If the field contents march one of the addresses associated with thestation, it strips the frame. If the bridge 15 had to compare thecontents of the frame source address field 43B with, for example, theaddresses of stations originating frames transmitted by the bridge todetermine whether to strip the frame, the bridge 15 would require veryfast, and therefore expensive, processors and memories. The same is truefor any stations which have many associated addresses, for example,stations which attach a relatively large number of devices to thenetwork. The inventive bridges 15-17 and stations 11 and 21 use theirinternal frame counts and strip delimiter frames to determine whichframes to strip, and thus they can use relatively simple circuitry andstate machines, to determine when to start stripping frames. i.e., whenthe frame count is non-zero, and when to stop stripping frames, asdiscussed below with reference to FIGS. 3A-C.

More specifically, with reference to FIG. 3B, when the bridge 15receives a bit stream over the ring 10, it first checks its internalframe count to determine if it is equal to zero (steps 70-71). If thecount is non-zero, the bridge 15 checks the information it is receivingover the network for the presence of a strip delimiter frame, the token,or a frame indicating a network error condition, that is, a claim tokenframe or a beacon frame (steps 72-74), any of which may result in thebridge not starting, or discontinuing, its frame stripping operation. Ifthe information is not any of the above, the bridge strips it, withoutcomparing the source address with any address list. The bridge 15 thendecrements its internal frame count by one for each error-free framestripped from the ring 20 (steps 76-77).

The bridge continues to so strip all the frames it receives on ring 20,until it encounters a delimiter frame, the token, or a claim token frameor a beacon frame, or its frame count is reduced to zero, for whateverreason, for example, during a frame stripping operation or duringstation re-initialization. The bridge may be receiving frames on a ringwhile it is in the act of transmitting frames on that ring. Thus thecount must be incremented and decremented such that the count willaccurately reflect the number of frames transmitted for which acorresponding frame has not yet been stripped, that is, if five frameshave been transmitted and three stripped, the count should be two. Inthe preferred embodiment the count is incremented and decremented at theend of each of the frames to ensure that the count is accurate.Accordingly, the count is never reduced below zero.

Errors in a frame may indicate a problem with a station 21 orcommunications link 22. Thus the bridge's internal frame count is notreduced for the stripping of an erroneous frame or frame fragments. Todetermine if a stripped frame is error-free the bridge 15 first checksthat the frame is a full, or valid, frame and not just a frame fragment.The count will not be decremented for a fragment. If the frame is avalid frame, the bridge then uses the frame check sequence 45 and theframe status flags 46B (FIG. 2) to check for errors.

Before the bridge 15 strips a frame from the ring 20, it examines theframe to determine if it is a strip delimiter frame, either its own orone from another station (steps 72 and 78). If the frame is anotherstation's strip delimiter frame, the bridge repeats it without alteringthe internal frame count (steps 78 and 82). The strip delimiter framethen travels around the ring 20 until it returns to the originatingstation 21 which strips it.

If the frame is the bridge's own strip delimiter frame, which signifiesthe end of the transmission, the bridge 15 strips it (steps 78-79). Ifthe strip delimiter frame is error-free the bridge 15 stops strippingthe frames it receives over the ring 20, even if the internal framecount indicates that all the transmitted frames have not been stripped,and it resets the internal frame count to zero (steps 80-81). The bridge15 will thereafter strip only its own strip delimiter frames or frameswith its station-address as the source address, repeating all the otherframes, until the internal frame count is again non-zero indicating abridge 15 transmission (steps 83-86).

The internal count is set to zero when the bridge 15 detects its ownstrip delimiter frame, as set forth above, to prevent the bridge 15 fromstripping frames originated by another station 21 on the ring 20. Thuseven if several of the bridge's frames are erroneously stripped byanother station 21, and its count is non-zero when it receives itsdelimiter frame, the bridge will correctly stop stripping.

Before stripping, the bridge also checks if the received information isthe token. Reception of the token indicates that the frames transmittedby the bridge have all been stripped, even the bridge's strip delimiterframes. The bridge will thus stop stripping and set its count to zero toprevent it from stripping another station's frames hereafter transmittedover the network.

The bridge 15 also stops stripping frames when the network is operatingin error detection and recovery mode, indicated by the receipt oferror-free claim token or beacon frame (step 90-96). If the bridge 15does not stop stripping frames during error detection and recoveryoperation, downstream stations would not receive the frames required fornetwork re-initialization. Thus, upon receipt of an error-free claimtoken frame or a beacon frame the bridge goes into error detection andrecovery mode operation as defined by the token ring protocol. Uponreceipt of erroneous claim token or beacon frames, the bridge 15 repeatsthe frames if the source address is not one of its own addresses orotherwise strips it, and then continues its frame stripping operation(steps 90-96).

Using the strip delimiter frame and the count to indicate when to startand stop stripping frames enables a network to be served by more thanone bridge, thus improving network throughput, flexibility andavailability. For example, if one path between a source station and adestination station is broken or congested, another path, connected byone or more bridges may be used. The scheme places no topologicalrestrictions on the networks, and thus, two networks may beinterconnected by redundant bridges.

Without using the strip delimiter frame and/or the count, a bridge onthe second path strips frames-based only on the source address, and ifit "recognizes" the source address in the frames as identifying one ofthe stations from which it normally copies frames, it may strip theframes, preventing them from reaching the intended destination. Thus, anetwork may not be served by redundant bridges. However, if the bridgeis using the strip delimiter frames and frame count, it will not stripthe frames, regardless of the source addresses in the frames, unless theaddresses match the bridge's own station-address. Thus there is no limiton the number of stations whose frames may be transmitted by a givenbridge or on the number of bridges in any network because framestripping is independent of the frame source address.

Using the strip delimiter frame to indicate the end of a transmission,as opposed to setting a flag or changing the contents of a frame fieldhas several additional advantages. For example, the strip delimiterframes uniquely identify their source station, while the frameindicators do not. Thus a strip delimiter frame which is not lost willeventually return to the originating station, signifying a potentialnetwork error condition if none of the other transmitted frames returnto the station. This occurs even if another station is in the process oferroneously stripping another station's frames when it receives thedelimiter frame.

Additionally, each strip delimiter frame may be error protected by itsown frame check sequence 45 (FIG. 2), while using the other indicatorsrequires the bridge to recalculate and replace the frame check sequence45 and/or other fields in one or more data frames. For robustness toerrors, multiple strip delimiter frames may be used to indicate the endof a transmission. In this way the bridge 15 is likely to receive onerecognizable strip delimiter frame even if one or more delimiter framesare lost or corrupted.

Using either the strip delimiter frame or the internal count aloneallows stations to correctly strip frames without reference to the framesource address, thus achieving some of the advantages set forth above.However, using both the strip delimiter frame and the frame count givesthe frame stripping operation an even further robustness to errors. Ifall the strip delimiter frames are corrupted or lost, the bridge willstrip frames only until its count is reduced to zero. Thus, at most, arelatively small number of frames which the bridge did not transmit willbe stripped. If only a flag in one of the transmitted frames is used toindicate the end of a transmission and the frame containing the flag islost or corrupted, many frames not transmitted by the bridge may bestripped.

Alternatively, the bridge may under-strip if only the flag is used toindicate the end of stripping. An error in one of the data frames mayresult in the frame's flag appearing set, the type of error which is nottypically protected by the frame check sequence, and the bridge thenstops stripping. Errors in the strip delimiter frames will not result inunderstripping because such erroneous frames do not signal the bridge tostop stripping.

It will be appreciated by those skilled in the art that any station, notjust bridge-stations, may use the internal frame count and/or stripdelimiter frames to determine when to start and stop stripping frames.Using the frame count and the strip delimiter frames enables a stationto use relatively fast and yet simple circuitry to control its framestripping operation, particularly if the station is associated with alarge number of addresses.

The foregoing description has been limited to a specific embodiment ofthis invention. It will be apparent, however, that variations andmodifications may be made to the invention, with the attainment of someor all of the advantages of the invention. Therefore, it is the objectof the appended claims to cover all such variations and modifications ascome within the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A station on a computer network, said stationcomprising:A. information frame transmitting means for transmitting oneor more information frames onto said computer network; B. stripdelimiter frame transmitting means, responsive to said information frametransmitting means, for transmitting one or more strip delimiter frameson to said computer network after said information frame transmission;B. strip delimiter frame detecting means for detecting strip delimiterframes on said computer network; and D. frame stripping means,responsive to said information frame transmitting means and said stripdelimiter frame detecting means, for stripping from said computernetwork information frames received after said information frametransmission is begun by said information frame transmitting means anduntil said strip delimiter frame detecting means detects one of saidstrip delimiter frames.
 2. A station on a computer network, said stationcomprising:A. transmitting means for transmitting one or moreinformation frames on the network; B. originating means responsive tosaid transmitting means for originating one or more strip delimiterframes and transmitting said delimiter frames on to the network at theend of the information frame transmission; C. strip delimiter framedetecting means for detecting said strip delimiter frames on thenetwork; D. token detecting means for detecting a token on the network;E. network error condition detecting means for detecting errorconditions on the network; and F. frame stripping means responsive tosaid transmitting means, and said strip delimiter frame detecting means,said token detecting means and said network error condition detectingmeans for stripping from the network information frames received afterframe transmission is begun and before:i. the detection of one of saidstrip delimiter frames is detected, ii. the detection of a token, oriii. the detection of a network error condition.
 3. A method forstripping frames from a computer network, comprising the steps of:A.transmitting one or more information frames onto said computer network;B. transmitting one or more strip delimiter frames onto said computernetwork after transmission of said one or more information frames; B.detecting error-free strip delimiter frames; D. stripping said one ormore information frames after said information frame transmission isbegun and until one of said one or more strip delimiter frames isdetected; and E. stripping said strip delimiter frames after said stripdelimiter frame transmission is begun and until all of said one or morestrip delimiter frames have been stripped.
 4. A station on a computernetwork, said station comprising:A. information frame transmitting meansfor transmitting one or more information frames on the network; B. stripdelimiter frame transmitting means, responsive to said information frametransmitting means, for transmitting one or more strip delimiter framesfollowing said information frame transmission; B. strip delimiter framedetecting means for detecting said strip delimiter frames on saidcomputer network; D. token detecting means for detecting a token on thenetwork; E. network error condition detecting means for detecting errorconditions on the network; F. frame stripping means, responsive to saidinformation frame transmitting means, said strip delimiter framedetecting means, said token detecting means, and said network errorcondition detecting means, for stripping from said computer networkinformation frames received after said information frame transmission isbegun and stopping said stripping upon:i. the detection of one of saidstrip delimiter frames by said strip delimiter frame detecting means, orii. the detection of a token by said token detecting means, or iii. thedetection of a network error condition by said network error conditiondetecting means.
 5. A station on a computer network, said stationcomprising:means for granting permission to said station to transmit atleast one information frame onto said network; means, responsive to saidgrant of permission to transmit, for said station to transmit at leastone information frame followed by at least one strip delimiter frameonto said network; and means for stripping frames from said network,beginning with said grant of permission to said station to transmit andending with receipt of at least one of said at least one strip delimiterframe.
 6. The apparatus as in claim 5 wherein said means for strippingframes from said network further comprises:means for stripping allframes arriving at said station from said network, regardless of anyaddressing information contained in said all frames arriving at saidstation.
 7. The apparatus as in claim 5, wherein said means forstripping frames ends stripping only upon receipt of an error free stripdelimiter frame.
 8. The apparatus as in claim 5 wherein said means forgranting permission to said station to transmit at least one informationframe onto said network further comprises:means for detecting that atoken was received by said station; and means, responsive to said meansfor detecting that a token was received by said station, for grantingsaid permission to said station to transmit at least one informationframe onto said network.
 9. The apparatus as in claim 5, said means forstripping frames further comprising:means for ending stripping upondetection by the station of a token.
 10. The apparatus as in claim 5,said means for stripping frames further comprising:means for endingstripping upon detection by the station of a network error.
 11. Theapparatus as in claim 5, said means for stripping frames furthercomprising:means for ending stripping upon detection by said station ofa beacon frame.
 12. The apparatus as in claim 5, said means forstripping frames further comprising:means for ending stripping upondetection by said station of a claim token frame.
 13. The apparatus asin claim 5, said means for stripping frames further comprising:means forending stripping upon detection by said station of a frame indicating anetwork error condition.
 14. The apparatus as in claim 5 furthercomprising:means for ending stripping upon detection by said station ofa beacon frame.
 15. The apparatus as in claim 5 further comprising:meansfor identifying said at least one strip delimiter frame as having beentransmitted by said station; and means, responsive to said means foridentifying said at least one strip delimiter frame as having beentransmitted by said station, for stripping said at least one stripdelimiter frame only if said at least one strip delimiter frame isidentified as having been transmitted by said station.
 16. The apparatusas in claim 15 wherein said means for identifying said at least onestrip delimiter frame as having been transmitted by said station furthercomprises:means for writing a source address of said station into asource address field of each said at least one strip delimiter frame.17. The apparatus as in claim 5 further comprising:means for writing anidentifying value into a frame control field of said strip delimiterframe.
 18. The apparatus as in claim 5 further comprising:means forwriting a source address of said station into a source address field ofsaid strip delimiter frame; means for writing an identifying value intoa frame control field of said strip delimiter frame; and means,responsive to a content of said source address field and responsive to acontent of said frame control field, for identifying a received stripdelimiter frame as having been transmitted by a different station, andfor forwarding said strip delimiter frame transmitted by a differentstation.
 19. The apparatus as in claim 5 wherein said means forstripping frames stops stripping frames from said network only uponreceipt of a strip delimiter frame transmitted by said station.
 20. Theapparatus as in claim 5 wherein said means for stripping frames stopsstripping only upon detection of an error free strip delimiter frame.21. The apparatus as in claim 20 further comprising:said detection of anerror free strip delimiter frame is made in response to a frame checksequence (FCS).
 22. The apparatus as in claim 20 further comprising:saiddetection of an error free strip delimiter frame is made in response toa frame status.
 23. The apparatus as in claim 5 further comprising:saidat least one strip delimiter frame is equal to exactly one stripdelimiter frame.
 24. The apparatus as in claim 5 further comprising:saidat least one strip delimiter frame is two strip delimiter frames. 25.The apparatus as in claim 5 further comprising:said at least one stripdelimiter frame is a plurality of frames.
 26. A station on a computernetwork, said station comprising:means for granting permission to saidstation to transmit at least one information frame onto said computernetwork; means, responsive to a grant of permission to said station totransmit, for transmitting at least one information frame onto saidcomputer network; means, responsive to said means for transmitting atleast one information frame for transmitting at least one stripdelimiter frame onto said computer network after said transmission ofsaid at least one information frame; means for stripping frames andframe fragments from said network, beginning with said grant ofpermission to said station to transmit and ending with receipt of atleast one of said at least one strip delimiter frame; means for writinga source address of said station into a source address field of each oneof said at least one strip delimiter frame; means for writing anidentifying value into a frame control field of said strip delimiterframe.
 27. The apparatus as in claim 26 further comprising:means,responsive to a content of said source address field and responsive to acontent of said frame control field, for identifying a strip delimiterframe transmitted by a different station, and for forwarding said stripdelimiter frame transmitted by a different station.
 28. A method foroperating a station on a computer network, comprising:grantingpermission to said station to transmit at least one information frameonto said computer network; transmitting at least one information frameonto said computer network; transmitting at least one strip delimiterframe onto said computer network after transmission of said at least oneinformation frame; stripping frames from said network, beginning withsaid grant of permission to said station to transmit and ending withreceipt of at least one of said at least one strip delimiter frame. 29.The method of claim 28 further comprising:stripping all frames arrivingat said station from said network, regardless of any addressinginformation contained in said all frames arriving at said station. 30.The method of claim 28 further comprising:stripping all frame fragmentsarriving at said station from said network, regardless of any addressinginformation contained in said all frames arriving at said station. 31.The method of claim 28 wherein said step of stripping frames endsstripping only upon receipt of an error free strip delimiter frame. 32.The method of claim 28 wherein said granting permission to said stationto transmit at least one frame onto said network further comprisesreceiving a token from said network.
 33. The method of claim 28 whereinsaid step of stripping frames ends stripping upon detection by thestation of a token, said detection occurring during stripping.
 34. Themethod of claim 28 wherein said step of stripping ends stripping upondetection by the station of a network error, said detection occurringduring stripping.
 35. The method of claim 28 furthercomprising:identifying said at least one strip delimiter frame as havingbeen transmitted by said station; forwarding a strip delimiter frameidentified as having been transmitted by a station other than saidstation.
 36. The method of claim 35 wherein said identifying said atleast one strip delimiter frame as having been transmitted by saidstation further comprises:writing a source address of said station intoa source address field of said at least one frame.
 37. The method ofclaim 28 further comprising:writing an identifying value into a framecontrol field of said strip delimiter frame.
 38. The method of claim 28further comprising:writing a source address of said station into asource address field of said strip delimiter frame; writing anidentifying value into a frame control field of said strip delimiterframe; identifying a strip delimiter frame transmitted by a differentstation in response to a content of said source address field an dinresponse to a content of said frame control field, and not strippingsaid strip delimiter frame transmitted by a different station.
 39. Themethod of claim 28 wherein said step of stripping frames stops strippingof frames from said network only upon receipt of a strip delimiter frametransmitted by said station.
 40. The method of claim 39 wherein saidstep of stripping frames stops stripping only upon detection of an errorfree strip delimiter frame.
 41. The method of claim 28 furthercomprising: stripping all frames arriving at said station from saidnetwork.
 42. The method of claim 28 further comprising:said at least onestrip delimiter frame is a plurality of strip delimiter frames.